1. Field of the Invention
The present invention relates to an apparatus and method for controlling the flow of compressed air, and in particular for controlling the flow rate of compressed air to a paint spray system and to other tools which are optimally operative dependent upon receiving a specified flow rate of air.
2. Background of the Invention
Many factories use compressed air as a source of power for operating various types of production equipment. xe2x80x9cCompressed air,xe2x80x9d which is sometimes referred to as xe2x80x9cpressurized airxe2x80x9d or referred in spray paint operations as xe2x80x9catomization air,xe2x80x9d is defined as free air that has been pressed into a volume that is smaller than it normally occupies. Controlled expansion of the compressed air can be used as a source of power to operate a wide range of pneumatically powered tools.
Compressed air is typically supplied from onsite or nearby compressors and piped through a distribution system to a downstream point of use. Paint spraying operations for painting various types of manufactured products, including especially automobiles and airplanes, is one typical use of compressed air.
In a spray paint operation, a paint fluid, which can be in the form of either a liquid or a fine powder, is mixed together with compressed air in a spray gun nozzle in order to atomize the paint into extremely fine particles and to transfer the paint particles onto the surface of the item being painted. One commonly used spray paint gun, referred to in the industry as a high volume low pressure (HVLP) spray gun, generates high volumes of low pressure air which transfers the paint particles to the surface of the article being painted with a relatively low velocity. Other uses of compressed air include pneumatically powered machine tools, drills and wrenches, and other pneumatically powered items which are optimally operative dependent upon receiving a specified flow rate of the pressurized air into the tool.
Major changes in downstream air demand create varying loads on the compressors. Air compressors are typically controlled according to system supply pressure, coming online as the system air pressure at the compressor drops below a threshold pressure and going offline at a higher cutout pressure. This is a rather crude method of controlling air supply, especially as air compressors are normally quite slow to respond to change.
Industrial compressed air systems are commonly controlled by pressure regulation, meaning, by regulating the nominal air pressure at a certain point in the system. A pressure regulator might be placed, for example, at or near the compressor, at one or more points on the distribution line, or on a hose which used to supply air to the tool. The major disadvantage with this method is that measuring air pressure at only one particular point in the system is not necessarily a good indicator of the air pressure at another point in the system. Air pressure drops as it flows through the system, and the amount that the air pressure drops from one point to the next varies greatly depending on the specific installation and also on varying conditions of usage during the course of the day. In many cases a compressed air system supplies not only spray guns but also other devices used in a paint shop such as sanders, polishers, screw drivers, drills and so forth. The intermittent operation of such other tools will affect the air pressure throughout the system.
For paint spray operations in particular, one commonly used method for determining whether a sufficient amount of compressed air is being delivered to the spray gun is to place a pressure gage on the cap of the spray gun immediately after the painter has set the spray gun for proper atomization of the paint but before he actually begins painting. Many operators, however, find this extra step to be a great inconvenience which interrupts their painting operations, and so they often do not do it. Another method to checking whether a sufficient amount of compressed air is being delivered to the spray gun is to attach an air pressure regulator and gage to the handle of the gun. However, attaching a pressure regulator to the gun naturally increases its weight. Over a period of time, muscle fatigue sets in, thereby causing the operator to use unnatural arm and wrist actions, which cause overspray or underspray conditions and other flaws in the paint job.
More importantly, regulating the nominal pressure at any one point in the system does not necessarily mean that the proper amount of air, or even any air is flowing at another point downstream. There may be a blockage in the spray nozzle of a paint gun, or a break in the line or some other problem in the system.
The difficulty in delivering a proper amount of compressed air to a spray paint nozzle is further exasperated by the fact that paint viscosity varies due to temperature fluctuations. If the temperature of the paint varies, the amount of paint fluid delivered to the nozzle of the spray gun also varies. Therefore, to compensate for the change in viscosity of the paint fluid, the amount of compressed air delivered to the spray gun nozzle must be adjusted. This type of adjustment is not easily done with only pressure regulation, and at a minimum requires a great deal of testing and trial and error to achieve the proper settings. Additionally, in many typical spray paint operations a single compressor is used to deliver compressed air to a number of output points. Each point is located a different distance from the air compressor, and so the pressure drops from the compressor to one point or the other will differ. Further, hoses which deliver air from the output point on the wall to the spray gun in the paint booth often differ in length, diameter or both, which greatly affects the pressure drop from one end of the hose to the other. Additionally, different types of spray paint guns, which require different amount of pressurized air, may be employed. The number of variables which are encountered during the course of operations thus increase to the point where it is nearly impossible to control the amount of compressed air delivered to the spray paint gun based merely on regulating the nominal air pressure at any particular point in the system.
Accordingly, an improved means for controlling the flow rate of compressed air delivered to a spray paint gun, as well as to other types of pneumatic tools which are optimally operative dependent upon receiving a specified flow rate of compressed air is desired.
A method and apparatus for monitoring and compensating the flow rate of compressed air delivered to a paint spray gun and other comparable pneumatic tools comprising both air flow rate and pressure based control of the compressed air system is disclosed. The invention includes an apparatus and method for measuring the air flow rate between two points in the system, comparing the measured flow rate to a desired flow rate, and then adjusting the flow rate in response to a difference between the measured flow rate and the desired flow rate, if any, and for also regulating the ultimate pressure in the system.
Generally, the present invention of an apparatus and method for controlling the air flow rate to a spray paint gun comprises: providing a source of compressed air; providing a source of paint fluid; mixing the paint fluid with the compressed air to thereby atomize the paint fluid and thereby transfer the atomized paint fluid to a substrate; measuring the flow rate of the compressed air by measuring a pressure differential across a fixed orifice located at a point downstream from the source of compressed air but upstream from where the compressed air is mixed with paint fluid; comparing the measured pressure differential to a desired pressure differential; and, in response to a difference between the measured pressure differential and the desired pressure differential, if any, adjusting the flow rate so that the measured pressure differential will equal the desired pressure differential. The above-described apparatus and method for controlling air flow rate is also preferably used in combination with a pressure regulating circuit that controls the ultimate pressure in the compressed air system particularly when the air flow rate drops to zero.
In the present invention, a pressure differential signal is generated by directing the compressed air through a fixed orifice obstruction, and by measuring the relative pressure difference between two points on either side of the orifice. The pressure differential signal is transmitted to a control device which compares the measured pressure signal to a desired pressure differential signal, and if there is a difference makes an automatic correction to adjust the flow rate to a desired amount. When the pressure differential signal is zero, which means that the air flow rate is also zero because the tool has been deactivated, the system automatically reverts to pressure regulation circuit. Thus, by controlling the rate of air delivered to the spray gun or other tool, as opposed to merely controlling the nominal air pressure at a single point in the system, the operator is assured that the proper amount of compressed air will be delivered to the tool. Additionally, when the tool is deactivated, the pressure regulation circuit assures that an overpressurization of the system will not occur.
The present invention of an air flow rate control method and apparatus provides a number of significant advantages over mere pressure regulation methods in the prior art. Such advantages include providing consistent air flow to the pneumatic tool regardless of supply hose diameter or length. The monitoring of flow rate can be used to detect various problems in the system such as pinched hoses, malfunctions of the tools, and other unexpected abnormalities in the demand or supply of the pressurized air.
Installations such as body shops normally include several air outlets so that the painter can disconnect the spray gun and move to various locations within the spray booth. Such installations now require several pressure regulators to be installed inside the spray booth. By using the air flow control of the present invention, a single supply pipe can be installed circling the spray booth area and the proper amount of compressed air will be delivered to the tool regardless of the location of the operator.
Other objects and advantages of the invention will become apparent from the following description which, taken in connection with the accompanying drawings, set forth by illustration and example certain embodiments of the present invention.